1. Field of the Invention
This invention relates to a Schmitt trigger circuit and more particularly to a Schmitt trigger circuit used as an input interface of an integrated circuit or the like.
2. Description of the Related Art
A Schmitt trigger circuit is designed with a weakened loop gain from positive feedback, so as to operate as a limiter. The circuit generates a rectangular-wave signal when triggered by a sinusoidal input signal of predetermined positive and negative voltage levels, using hysteresis of the circuit for selection.
A typical, conventional Schmitt trigger circuit in a complementary MOS transistor integrated circuit (hereinafter called "CMOS") is composed of an input resistor, two series connected CMOS inverters and a resistive feedback loop. In such a circuit, hysteresis is set according to a ratio of resistances of the input and feedback resistors. If the amplitude of a sinusoidal input signal applied to the input terminal exceeds the threshold voltage of the first inverter upon reaching its input node, the signal is inverted by the first inverter. If the level of the inverted signal exceeds the threshold voltage of the second inverter, the signal is again inverted. The output of the second inverter is fed back to the input node through the feedback resistor. Therefore, a rectangular-wave output signal is output from the second inverter.
In this type of Schmitt trigger circuit, when the threshold voltages of the CMOS inverters are set at 1/2 of the supply voltage, levels of hysteresis of the circuit are easily set by the resistance ratio. However, as seen through the input terminal, the input and feedback resistors form a resistive path. This circuit therefore has high current consumption.
Another known Schmitt trigger circuit has been proposed to solve the problem of high current consumption. This Schmitt trigger circuit substitutes capacitors at the positions of the input and feedback resistors, so that the impedance thereof is high.
In the operation of such a Schmitt trigger circuit, the voltage of the input signal drops across the input capacitor in proportion to the ratio of the impedance of the input capacitor to the sum of the respective impedances of the input and feedback capacitors. Since capacitive impedance is inversely proportional to capacitance, it is possible to increase the impedance by reducing the capacitance. Therefore, when this Schmitt trigger circuit is provided in an integrated circuit, an increase in impedance, and a corresponding reduction of capacitance, can contribute to a designed shrinkage of the integrated circuit.
However, this circuit presents problems as well. In the case where the input signal is in a low frequency band, i.e. a direct current region, the impedance of the input capacitor may become so high that the input signal cannot be transmitted through the circuit. That is, the voltage on the input node of the first inverter may be reduced to such an extent that the Schmitt trigger circuit cannot operate properly. Thus, the circuit cannot transmit signals in a direct current region. In addition, this circuit is prone to malfunction when the input signal contains noise.
Therefore, these circuits do not satisfy a need for a Schmitt trigger circuit having a lower current consumption, sufficient characteristics to permit operation without malfunction even when the input signal contains noise, and adequate operation in the direct current region.